The annual incidence of myocardial infarction is nearly one million and yet no therapy currently exists to restore lost muscle function following an attack. Stem cell recruitment and/or transplantation are potential strategies for boosting the regenerative potential of the heart. However recent experience has taught that stem cells are hesitant to engraft and adopt the functional traits of cardiomyocytes. This proposal is based on our finding that transplanted stem cells fuse with cardiomyocytes in healthy cardiac tissue and contribute to the function of the heart of xenogeneic chimeras. We hypothesize that myocardial repair following infarction could be enhanced by induction of fusion between donor stem cells and recipient cardiomyocytes. Cell fusion in this context would promote engraftment and drive differentiation of stem cells to functional cardiomyocytes. We propose to co-opt the unique fusion capabilities of the Moloney murine leukemia virus (MuLV) to develop a method for increasing the frequency of fusion between stem cells and cardiomyocytes. MuLV encodes, among other products, the envelope (env) proteins SU (surface molecule, gp70) and TM (transmembrane molecule, p15E). These proteins form a complex which mediates the binding of two liposomal membranes and in so doing, promotes the intermingling of the cytoplasmic components contained in the membranes. In Aim I, we propose to evaluate the in vitro efficacy of gp70/p15E-induced fusion between mesenchymal stem cells (MSCs) and cardiomyocytes (CM). This will be accomplished by isolating bone marrow-derived MSCs and cardiomyocytes from wild type mice. We will transfect MSCs with a vector encoding murine endogenous retroviral fusion proteins (MuLV gp70 and MuLV p15E) based on conditions found to maximize the frequency of fusion and minimize immune responses of syngeneic leukocytes. For efficacy studies, we will determine the viability of fusion products and their ability to maintain sarcomeric organization and baseline levels of muscle function. In Aim II, we will treat myocardial infarct zones with gp70/p15E-producing MSCs. We will induce acute myocardial infarction in mice and test the effects of a single infusion of gp70/p15E-transformed syngeneic MSCs on myocardial regeneration. We will monitor the extent to which injected MSCs fused with resident cardiomyocytes and identify both the beneficial and adverse effects of cell fusion via live cell tracking, phenotypic and functional assessment of the cells of ventricular cross-sections, determine the composition of the extracellular matrix of the myocardium of the ventricle and assess time-dependent changes in left ventricular geometry and function. We expect that MSC-CM fusion will promote engraftment of MSCs and differentiation of MSCs which will in turn promote regeneration of the injured myocardium. PUBLIC HEALTH RELEVANCE: Myocardial infarction (i.e., heart attack) occurs in nearly one million individuals every year and yet there exists no therapy to recover lost heart function after such an event. Stem cells could provide a basis for effective therapies to recover lost heart function. We propose to utilize insights gained from stem cell function in healthy tissue and tools optimized by viruses to advance the utility of stem cells for repair of the heart following an attack.